In the printing field, there has been much effort devoted to printing at higher speeds while, at the same time, maintaining print quality. As is well-known, a plurality of character fonts are utilized by businesses for various purposes, although there is a tendency to standardize the number of fonts so as to decrease costs of printing and to provide product compatibility between manufacturers of business equipment. As is also well-known, the surface area of characters to be printed varies substantially according to the number of different characters. For example, in impact printing mechanism for numerals 0-9 it is desirable that for each and every printed character, the quality of the printing should be uniform regardless of the surface area of the character.
In the past, all the characters have been grouped into two categories of high or low surface area, or at the most, into three or four different character areas or energy levels with the print quality being accepted for economic or technical reasons. Additionally, each energy level has usually required an independent adjustment of the impact hammers to obtain a nearly uniform surface area print quality.
Printing mechanism concerned with different surface area of characters is disclosed in U.S. Pat. No. 2,935,935, issued on May 10, 1960 to W. C. Preston et al., wherein a series of printing hammers are capable of delivering different impact energies on the various characters by providing means for controlling the amount of pressure that is applied to each print hammer.
Representative prior art of a variable force hammer high speed printer is U.S. Pat. No. 3,172,353, issued on Mar. 9, 1965 to C. J. Helms, which discloses magnetic braking and intensity control for low surface area and high surface area characters. A photosensitive element reads, by means of a code wheel, information identifying whether each character is a high or low surface area. Each character on the typewheel is represented by a six bit binary code which can be read from the code wheel by elements which are connected to a computer in which is stored the information to be printed. The computer compares each successive code read by the elements with the information to be printed and provides output signals which cause selected hammer coils to be energized for each position of the drum. The computer can provide either encoded address signals identifying particular hammers, or decoding means within the computer for providing an output signal on each line to a hammer to be actuated. Circuitry is provided to energize the coils at a certain time to cause information to be printed in accordance with information stored in the computer.
U.S. Pat. No. 3,218,965, issued on Nov. 23, 1965 to J. C. Simons et al., shows pressure control means for print hammers wherein a variable energy is provided to each print hammer dependent upon the surface area of the character to be printed. Each rack is positioned a differential distance according to its respective character, and a character area arm has indentations according to the surface area of the character to be printed. An energy sensor is actuated to sense which character is to be printed, the sensor having a projection to sense the indentation. Spring anchors, directly coupled to the springs which provide the driving energy for the hammers, are positioned according to energy sensors and a crank releases the hammers to print the respective characters.
U.S. Pat. No. 3,308,749, issued on Mar. 14, 1967 to A. A. Dowd, discloses print impression control for specially configured type elements wherein the rear surface of the print type element immediately behind the embossed character is inclined relative to the striking surface of the print hammer, the effect being that the impact force gradient corresponds to the surface area distribution of the character formed on the type element.
U.S. Pat. No. 3,513,774, issued on May 26, 1970 to J. P. Pawletko et al., discloses print hammer compensation effected wherein the type train velocity and the source voltage have timing pulses from an emitter driven in synchronism with a type chain applied to a single shot multivibrator and a filter to develop a velocity error voltage. This voltage is used as a reference for a constant current ramp connected to a Schmitt trigger for developing a velocity error time-corrected timing pulse, the pulse being applied to a voltage correction circuit for developing a further timing pulse, which is both velocity and voltage error time compensated.
And, U.S. Pat. No. 3,712,212, issued on Jan. 23, 1973 to J. Beery, discloses variable printer intensity control wherein a control circuit is associated with printing apparatus, viz. a document encoding station including an impact print member cooperable with a character bearing cyclically movable member. The control circuit includes gate means which, depending upon the surface area of the selected character to be printed, can modify the current supplied by the circuit to the energy producing means for impacting the hammer.